First, you absolutely must use the dungeon finder to queue for the world boss. If you just go into a random dungeon or go into the dungeon directly the boss will not be there.

Secondly, the boss will no longer have a chance to drop the incredibly rare vanity items such as the Big Love Rocket or the Horseman's Reins. Instead, those will drop out of a special holiday themed loot trove that each person will receive the first time they kill the holiday boss on a given day, much in the same way one receives 2 frost emblems for the first random heroic they get.

Finally, Since there's no summoning the boss, you can't bring in alts to get multiple summon chances, but you can re-queue to kill it as many times as you want. However, you'll only get the extra holiday loot bag once. Note that you'll still get the more "common" drops such as trinkets and rings and the like every single time.

In the short run, this would appear to greatly lower a person's chance at getting the rare mount, as they get exactly one shot at it per day. On the plus side, though, they will never have to roll against anyone else for it, and there's no chance of being "cheated" out of their chance at it by someone who joins a pickup run despite having already spent their summon for the day. It could be that the loot caches will even have a higher chance to drop the rare item, though we probably won't know for sure until we've had a chance to experience the full holiday and see how the loot drops go. Either way, this is a heads up: The way you hunt down holiday event bosses is going to change pretty drastically come Patch 3.3.3. Be ready.